Receiver selectivity control



M. G. CROSBY RECEIVER SELECTIVITY CONTROL April 2, 1946.

Filed' July 2, 1943 2 Sheets-Sheet l INVENTOR.

MUR/4 Y 6. CROSBY vBY Mmm/EY April 2, 1946. M. G. cRosBY RECEVER SELECTIVITY CONTROL Filed July 2, 1943 2 Sheets-Sheet 2 IN VEN TOR.

MURRAY G. CROSBY BY #QZ ATTORNEY Patented Apr. 2, 1946 RECEIVER SELECTIVITY CON TROLV Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 2, 1943, SeralNo. 493,247

(Cl. Z50-20) 7 Claims.

My present invention relates to reactance tube control circuits for radio receivers, and more particularly to self-adjusting selectivity control circuits for angle modulated carrier wave receivers.

An important object of my invention is to provide a resonant reactance tube circuit capable oi automatically shifting the frequency of a selector network of a receiver of angle modulated carrier wave energy so as to pass efliciently all frequency components in the received band of signal energy. v

Another important object of my invention is to provide a novel method of frequency-shiiting a tuned selector circuit of a receiver of frequency modulated carrier wave energy.

Another' object of my present invention is to accommodate automatically, and with simple and few elements, the pass band of -a tuned acceptor circuit for angle modulated carrier waves to the frequency swing of the waves.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the `invention itself, however, as'to bothits organization and method of operation will best be understood by reference to the following description, taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. v1 shows one embodiment of the invention, as applied to the intermediate frequency (I. F.) amplifier of a frequency modulationreceiver of the superheterodyne type,

Fig. 2 shows in idealized form the phase Shifting characteristic of the tuned transformer of the reactance tube,

Fig. 3 illustrates ideally the effect of the reactance tube circuit on the controlled selector circuit characteristic,

Fig. 4 shows a modification of the invention.

Fig. 5 shows graphically the ideal functioning of the circuit of Fig. 4,

Fig. 6 illustrates still another modification of the invention.

In considering the embodiment of the invention shown in Fig. 1, it is to be assumed that the tube l is an I. F. amplifier tube of a superheterodyne receiver adapted to receive angle modulated carrier wave energy. The latter term angle modulated carrier wave energy is gehybrids thereof. By way of specific example, it is assumed that the I. F. arnpliiier stage shown in Fig. 1 is utilized in a superheterodyne receiver of FM waves. The receiver may be tunable through the assigned FM band of LlZ-tli) megacycles (ma). However, the invention is not restricted to this particular frequency band. In the l2-50 mc. band each station carrier, or mean, frequency Fc has a maximum deviation of substantially '75 kilocycles (kc.) to each side of Fc. The frequency deviation ofthe mean frequency is proportional to the amplitudeof the modu1a tion signals, while the rate or frequency deviation is representative of the modulation frequencies per se.

As vis well known to those skilled in the art, the conventional superheterodyne receiver includes tunable selector circuits arranged in the amplifier stages preceding the converter network. The function of the converter is to reduce the mean frequency of the selected and amplified FM signal energy to the. operating I. F. value. The latter may be chosen from a range of frequencies covering- 4 to 25 mc., and preferably a value of 8.33 mc. is employed as a satisfactory compromise value. The various selector circuits up to and including the input transformer 2 of tube l have each a pass band width such as efliciently to pass the overall frequency swing of the selected FM signal energy. Of course, each selector circuit prior to tube I may be replaced by a controlled selector circuit of the type forming the subject matter ofmy present invention." Each of the primary and secondary circuits of input transformer 2 is tuned to the operating I. F. value. The tube I may be of any conventional form, and the circuit details thereof are well known and need not be described in further detail.

The plate circuit of tube l includes a resonant circuit which consists of coil 3 shunted by condenser 6. The resonant circuit 3-4 is tuned to the operating I. F. value, and has a resonance curve or characteristicwhich has a single peak. The FM signal'energy developed across the resonant vcircuit 3 4 is transmitted through the coupling condenser 5 to the following I. F. amplifier stage. The latter may be a stage similar to the one shown in Fig. 1, or it may be the amplitude limiter employed prior to the FM detector circuit. The function of the limiter is substantially to remove any amplitude modulation on the modulated carrier energy. The FM detector circuit derives the modulationsignal energy fronrthe limited signal energy fed to the detector, Y

Y operating I. F. value.

According to my invention, the selector circuit Q is automatically shifted in frequency to accommodate the frequency swing of the received frequency modulated carrier wave energy. It is, again, emphasized that my present explanation is restricted to the selector circuit 3 6 merely by way ofV illustration, and that the same functions may be performed in connection with other selectory circuits of the receiving system prior to the FM detector circuit. In accordance with my invention, I utilize an electron discharge tube S, which is termed a reactance tube since it functions to simulate a reactive effect across-the tuned selector circuit 3 4.

The plateY 'I of tube 6 is connected to the plate of amplifier l. Hence, the -l-B terminal of the direct current voltage supply (not shown) is connected not only to the plate of the I. `Ramplier tube I, but also to the plate 1 of the reactance tube. The cathode of tube 6 is connected vto ground through a carrier-bypassedbiasing ree sister S. It will, therefore, be seen that the plate to cathode impedance of tube 6 is connected in shunt with the selector circuit 3 4. The reactive sign and magnitude of the plate to cathode im pedance of tube 6 determine the direction and extent of. frequency shifting of selector circuit 3 4.v The reactive sign and magnitude of the plate to cathode impedance of tube 6 are determined by the circuits now to be described.

n A transformer 9`is provided for coupling the plate 'l to the control grid I0 of tube 6. The primary circuit of transformer 9 is designated by numeral il. The circuit Il is tuned to the mean frequency of the applied FM wave energy, this mean frequency, indicatedby the symbol Fc, is thev The low potential side of circuit Il is grounded, while the ungrounded side is connected to plate 'l by the coupling condenser I2. The reactance of condenser I2 is adjusted to be high incomparison to the input resistance of transformer 9. The secondary circuit i3 of trans-4 former 9 is likewise tuned to FaranditS 11ngrounded side is connected directly tothe controly grid l0. The circuits vIl and i3 are indicated as being magnetically coupled, and each of them is shunted by a damping resistor in order to provide a normal band-pass characteristic for selector circuit l l l3 at the mean frequency Fc. The numerals il' and I3' designate the damping .re-.- sistors of circuits Il and I3V respectively.

The alternating voltage between plate 1 and ground is represented by the symbol E1; the

alternating voltage developed across the primary circuit il is represented by the symbol Ez; and the alternating Voltage between control grid I and ground is represented by the symbol Es.' At the mean frequency Fc the plate to cathode im` pedance of tube 6 functions in the manner of a resistive impedance which provides a predeter-` mined degree'of damping for the selector circuit 3 4. In Fig. 3 I have indicated by the central resonance'curve, shown in solid line, the relative appearance of the selector resonance charactertransformer primary is determined by the coupling condenser so that the current leads the applied voltage. i The voltage drop across the transformer primary will be in phase with the current, since the transformer primary is resistive. This voltage drop will, therefore, lead the voltage applied from plate 1. An additional phase shift is imparted to the feedbackY voltage of the reactance tubeby means of the transformer 9. It is well known that such a, tuned transformer imparts a phase shift of 90 degrees at the midband frequency. This results in voltage E3 being approximately in phase with the voltage E1 applied from plate 1. The overall phase shift characteristic of the coupling condenser and transformer is as shown ideally lin Fig. 2. At the carrier, or midband, frequency the phase shift is zero, while deviations from the carrier frequency produce leadT ing or lagging phase shifts depending on'the diwith frequency is caused by the inherent characteristic of the .tuned transformer. Since the feedback of the reactance tube is resistive at the mean frequency Fc, the reactance tube presents a resistive plate, circuit across selector circuit 3 4. This producesa damping action which broadens the selectivity of the selector circuit, as shown by the solid line central curve of Fig. 3. Adjusting the voltage of an electrode of tube 6, for example the screen, gives a Ameasure of control over the band width of circuit 3 4. Y

As the instantaneous frequency of the applied FM signal energy shifts in an increasing direction towards the maximum frequency point, the plate to cathode impedance of tube 6 has an inductive reactance which increases in magnitude in accordance with the extent of ,the frequency deviation from mean frequency. At the same time the shunt resistance component is increased so as to reduce the degree of damping. This follows from the fact that if the resistance component across circuit 3 4 increases, the equivalent` series resistance in circuit 3 4 decreases. 3 4 will be relatively sharper at such times.

Y Conversely, theplate to cathode impedance of tube 6 simulates a capacitative reactancev as the instantaneous. frequency shifts to frequencies less than the mean frequency, while the'shunt resistance component is likewise increased(Y In Fig. 3 there is shown the effect of the inductive reactance X1. on the resonance curve configuration and frequency location of selector circuit 3 4. Similarly, Fig. 3 shows the effect of the capacitative reactance Xe on the selector circuit 3 4. These eects are shown in Fig. 3 by the dotted line resonance curves located on opposite sides of the central solid line curve. It will be seen that the eiect of each of the inductive reactance and capacitative reactance is to shift the resonance curve in respectively opposite directions relative to the mean frequency. Further, the coniiguration of the shifted resonance curve is relatively sharper than at the mean frequency for the reasons explained in the preceding paragraph. Hence, there is produced an overall relatively wide acceptance curve fprthe selector circuit` 3 4.l and this widening of theA acceptance curve permits the selector circuit 3 4 automatically to be accommodated to the extent of the overall frequency swing of the FM signal energy. The resultant wide acceptance curve in Fig. 3 is repre- K sented by the dotted line between the peaks of the extreme sharpened vresonance curves. Furthermore, since the shifting of the resonance curve is Hence, the resonance curve of circuit performed 'in response to the instantaneous value of the applied FM signal energy, it will be realized that the accommodation occurs at a very rapid rate.

It will be notedV that the overall wide response curve shown in Fig. 3'is not flat-topped, but dips at the carrier frequency due to the variable damping action of the resistive component cf the reactance tube. It will be apparent to those skilled in the art that the overall response may be adiusted to a fiat-topped characteristic b-y adjusting any other selector in the system, for instance transformer 2, to be round-topped to a compensating degree. Y

It will now be appreciated that instead of the usual wide band-pass transformers provided in the various selector circuits prior to the FM detector there may be used the controlled selector circuits of the present invention. When the frequency swing of the applied FM signal energy is relatively narrow, the selector circuit 3 4 will have its frequency shifted to a relatively small extent. On the other hand, when the frequency swing of the applied signal energy is very wide the accommodation of the selector circuit 3 4 will be correspondingly increased. Furthermore, it is apparent that this frequency-following effect is not present for the case of amplitude modulation which is undesired in angle modulation reception.

Consequently, such a receiver has the property of Fig. 4 I have shown a different form of tuned phase-shifter circuit. In Fig. 4 the amplifier tube i has its input circuit tuned to the mean frequency Fc of the applied signal energy. Its cutput circuit 3 4 is tuned to the same mean ,frequency Fc. The reactance tube has its plate to cathode impedance connected in shunt across the output circuit 3 4. There is, also, connected in shunt across the output circuit 3 4 a phase shifter network which includes a series resonant circuit tuned to the mean frequency Fc. The phase shifter network is indicated within the dotted rectangle, and consists of the series resonant circuit zii-2 i. The lower end of coil 2! is grounded, while the upper terminal of condenser is connected directly to the control grid I0 of tube 6. The grid IB is returned to ground through a direct current return resistor 22 whereby normal negative bias, developedacross biasing resistor 8, is applied to grid l0.

The plate 7 of the reactance tube is connected to the phase shifter network by the direct current blocking condenser 23 and the series resistor 24. The type of reactance tube network shown in Fig. 4 functions to sharpen the tuning of output circuit 3-4. If the instantaneous yfrequer'icy of the applied.v signal energy at input transformer 2 increases above the mean frequency Fc. then the series resonant circuit 28 2| has an inductive effect. At the' mean frequency Fc, of course, the

series resonant circuit is essentially resistive.

Where the instantaneous frequency of the applied signal energy drops below` the mean frequency Fc, the series resonant circuit appears as a capacitative reactance. The result is that the effective .frequency of circuit 3 4 is tuned in a direction opposite to the signal frequency deviation.

Fig. 5 shows how the selectivity is made greater by this self-tuning action. Curve A A is the selectivity curve of tuned circuit 3 4 in the absence of reactance tube 6. vThe' operation ofthe reactance tube is rsuch as to shift the tuning of 3 4 towards a higher frequency, such as is shown by curve C C, when the frequency of the applied signal instantaneously shifts towards a lower frequency. Likewise, the tuning shifts lower when the signal frequency shifts higher. This oppositedirection self-tuning action is brought about by using av tuned phase-,shifter circuit between the plate and cathode of the reactance tube such that the reactance tube gives a capacity effect for frequencies higher than thecarrier or mean frequency, and an inductive effect for frequencies lower than the mean frequency. Thus, when the carrier frequency shifts from the frequency Fc to the frequency F1, the amplitude changes from the point u on curve A Ato point e on curve1B B. When the frequency shifts lower to F2, the amp1itude drops from u on A A to w on curve C C. It is thus apparent that the resulting selectivity curve will follow the curve D D, which is consid erably sharper than the original characteristic which existed in the absence of the reactance tube.

The selectivity of circuit 3 4' can then be varied, if desired, by varying the mutual conductu ance of reactance 'tube 6. As is well known to those skilled in the art, variation of the bias of control grid I0 will result in variation of the gainl of the reactance tube. Accordingly, the effective magnitude of the simulated capacitative or inductive reactance can be controlled.

In Fig. 6 I have shown a modification of the invention wherein the phase shifter network which is operatively associated with the reactance tube 45 is aperiodic. The tube 40 is controlled in sense of eifect and magnitude of effect by means of the usual form of discriminator-rectiiier employed in frequency modulation receivers. Let it be assumed that numeral 4| denotes the usual and conventionalv form 0f I. F. amplifier used in a superheterodyne receiver of frequency modulated carrier waves. The'plate circuit of the amplifier fil includes the parallel resonant circuit 42 43 which is tuned to the operating I.F. value. The plate to cathode impedance of reactance tube 4G is connected effectively in shunt across the resonant output circuit 43 42 of the I. F. amplifier 4i. The `reactance tube 40 is made tor simulate a variable capacitativelreactance, as is indicated by the dotted condenser 44 shown 'in shunt across coil 42.

This simulated reactance effect is produced by the phase shifter 'shown in the dotted rectangle. The phase shifter inthis modification is aperiodic, since it consists of condenser 45 connected in series with resistor 46 between the plate of tube fit and ground. The ungrounded end of the resistor 48 is connectedv to the control grid 41. By making the resistive impedance of resistor 46 small compared to the capacitative reactance of condenser 45 the alternating voltage applied to the control grid 41 will be in phase quadrature relative to the alternating potential at the plate of reactance tube 4i). Withthese relations the plate to cathode impedance of tube 4G appears as if it were a capacitative reactance across the output circuit 43-42. v

Of course, the capacitative reactance has a normal predetermined magnitude at the` mean frequency Fc of the applied signal energy. In order to Varythe magnitude of this reactive effect in stantaneous frequency deviations from the mean frequency are translated into control voltages whose polarity and magnitude are a function respectively of the directionand amount of instantaneous frequency deviation.v This is done by employing a conventional FMdetector circuit which may include a pair of opposed diode rectiiiers 50 and 5I'. The. resonant input circuit 52 is connected between the anode and cathode of rectiner 59, and the carrier-massed resistorv53: acts as the load resistor of the. rectier- 5B. vIn the same way, the input circuit. 54 is connected between the anode and cathode of rectier 5l.. The' carrior-bypassed resistor 55 acts as the. load resistor of rectier 5I. The junction of resistors 53Y and 55. is connected 'to the common junction cf the two input circuits 52: and 54. y

The cathode of diode 5l isgrounded.. The differential voltage developed across resistors 5.3 and 55 iny series is taken off from Ythe cathode end of resistor 53. Input circuits 52 and 5.4.- are oppositely and equally-.mistuned ,with respect to the operating I. F. value,v as is well known to those skilled in the art, with the result that the desired discriminator characteristic. is provided for the opposed rectiers. A limiter 611 is interposed between the resonant output circuit- 432-42 and the common signal input. circuit 6.1 which feeds the discriminator circuits. The` function of the limiter is substantially to eliminate any amplitude lmodulation effects which may appear on the carrier. wave. The inputY circuit. El will. be tuned to the mean frequency of the applied signal energy,

yand is, therefore, tuned to a frequency which is midway between `the spaced resonant frequencies of circuits 52 and 5.4., The circuit 6l. is magnetically'coupled to each, of.V the oli-tuned. circuits 52 and 54.

The audio modulation voltagedeveloped at the cathode end of resistor 513 is transmitted to an audio utilization circuit. .On the other hand, ,the direct current voltage developed across the series related resistors 53 and 55 is fed to a control grid la of tube lle. The control grid 'ill isan intermediate grid of the. tube, and itis given a normal negative bias determined by the voltage drop across the cathode biasing resistor 7B arranged in the cathode circuit ofthe reactance tube At the instant when the frequency of the'signal g energy applied to. circuiti! is equal to the mhean frequency Fc, the rectified voltage across resistors 53 and 55 will be substantially zero. This follows from the fact that the opposed rectiers will have equal outputs developed across their respective lload resistors. However, as the instantaneous frequency of the signal energy at circuit 6l deviates from the mean frequency, the fact that the input circuits 52 and 54 are oppositely and equally mistuned relativel to the mean frequency will cause the potential at the cathode end of resistor 53 .tofollow in polarity and magnitude the direction and extent of instantaneous frequency deviations from the mean frequency. The rectied voltages developed across each of resistors 53 and 55 are, in other Words, combined difierentially,y and the diierential voltage applied over lead all to the grid 1.0 of reactance tube all.

Hence, it willjbe seen that the mutual conduct--y i ance of tube 4U will be varied in 'response to the potential of the cathode end of 53. This potential will be-negative or positive relativev to ground dependingupon whether the instantaneous frequency of the signal energy at circuit 6| deviates toward the frequency of circuit 54 or deviates toward the frequency of circuit 52 respectively. By way of specific example, assume circuit 52 is ltuned below Fo while. circuit 54 is tuned above Fc.

accueil2 Since it is desiredto vary the resonance curve characteristic of circuit 45t-*tt in response tothe instantaneousV changes of: the frequency of the received signal energy, it is important that there be omitted :from lead 8.o anynlterins of the rectified modulation voltage.. It is this modulation signal voltage which; causes the simulated reactance effect 4B to follow the frequency deviation of the received signal. energy.

While I have indicated and described several systems for carrying my invention into effect, 'it will be apparenty toone skilled in the art that my invention is by no means. limited to the particular organizations shown and described, but. that many modifications may he made without departing from the-scope of my invention.

What I claim is:

1. In combination with a. source of angle. modulated oarrier wave ener-gyz a. resonant selector cire f cuittuned normallyto the mean frequency of the ansie modulated wave energy. an electron discharge tube provided withY at least a cathode, a control. grid and an anode, means connecting the anode to cathode impedance, Of said tube in shunt with said selector circuit, a. resonant phase shifter network connected between the anodeand cathode of said tube, said phase shifter being normally tuned to said mean frequency, and means for applying the alternating voltageacross said shifter network between the control grid and cathode. e

2. In combination with a source of angle modulated carrier wave energy. a resonant selector circuit tuned normally to the mean frequency of the angle modulated wave energy, an yelectron discharge tube provided with atleast a cathode, a control grid and an anode', means, connecting the anode to cathode impedance of said tube in shunt with said selector circuit, a resonant phase shifter network connected between. the anode and cath.-

ode of said tube, said. phase shifter being nor'- mally tuned to said mean frequency, said resonant phase shifter consisting of a pair of reactivel-y coupled tuned circuits arranged in cascade between said, anode. and control grid.

3. In combination with a. source. of angle. modu lated carrier wave energy, a resonant selector circuit tuned normally to themean frequency of th angle modulated wave energy, an electron discharge tube provided with at least acathode, a control grid and an anode, meansjconnecting the anode to cathode impedance of Said tube in shunt with said selector circuit, andy a resonant phase shifter network connected between the anode and cathode of said tuber said phase shifter being normally tuned to. said mean frequency, said phase shifter consisting of a series resonant crcuit connected between said controlV grid and a point at ground potential. .f

4. In combination with a source of angle modulated carrier wave energy, a resonant selector circuit tuned normally to the. mean frequency of the angle modulated wave energy, an electron dis- Charge tube provided with at least a cathode,l a control grid and an anode, means connecting the anode to cathode impedance of said tube. in shunt with said selector circuit, aY phase shifter Vnormally tuned to said mean frequency, means for applying alternating voltage at said anode, to said phase shifter, said tuned phase. shifter consisting of a resonant circuit tuned to said mean frequency and connected between said control grid and a point at ground potential.

5. In combination, in a frequencymodulatlon receiver, an amplifier tube having a selective input circuit tuned to the mean frequency of applied frequency modulation signal energy, a selector circuit connected to the output electrodes of said amplifier tube, said selector circuit being tuned to said mean frequency, an electron discharge tube having its anode to cathode impedance operatively associated with said selector circuit, a resonant phase shifter network tuned to said mean frequency, said phase shifter network being connected in shunt with said impedance, and means responsive to signal voltage developed across said phase shifter for controlling said impedance for automatically varying the tuning of said selector circuit in accordance with the instantaneous frequency deviations of the signal energy with respect to said mean frequency.

6. In combination with a source of angle modulated carrier wave energy, a selector circuit tuned normally to the mean frequency of the angle modulated wave energy, an electron discharge tube provided with at least an anode, cathode and control grid electrode, means connecting the anode to cathode impedance of said tube in shunt with said selector circuit, a resonant phase shifter network connected between the anode and cathode of said tube, means for applying the alternating voltage across the phasev shifter network to said control grid, said phase shifter being normally tuned to said mean frequency, and means for varying an electrode voltage of said tube to adjust the effect of said impedance. Y

7. In combination with a source of angle modulated carrier wave energy, a resonant selector circuit tuned normally to the mean frequency of the angle modulated wave energy, an electron discharge tube provided with at least a cathode, a control grid and an anode, means connecting the anode to cathode impedance of said tube in shunt with said selector circuit, a resonant phase shifter network connected between the anode and cathode of said tube, said resonant phase shifter consisting of a pair of reactively coupled tuned circuits arranged in cascade between said anode and control grid, each of the pair of circuits being tuned to the mean frequency thereby to cause the impedance to be capacitative or inductive for frequency deviations of the energy.

MURRAY G. CROSBY. 

